The invention relates to a centering rest, as for location of a rotary axis of symmetry of a workpiece. So-called "tailstocks" or "center rests" are used on lathe and the like machine tools and on measuring devices, to provide support for rotatably chucked cylindrical workpieces such as gearwheels. The center rest clamps at least one axial end of the workpiece at a center (point) in the axis of rotation of the workpiece. In such supports, the receiving center (hereinafter sometimes referred to as the "opposing center") is axially opposite the rotary drive and is displaceable in the direction of the axis of rotation, to permit the clamping of workpieces of different axial length. As a rule, the tailstock or opposing center is carried by an arm which is longitudinally guided at offset from the axis of rotation and parallel thereto.
Since spaced centers determine location of the rotary axis of the workpiece which is to be machined or measured, the work-engaging center, which is carried by the displaceable arm of the tailstock, should lie, for all clamping distances, in exactly reproducible manner on a predetermined straight line. In this connection, permissible tolerances must be within the micrometer or submicrometer range.
However, such a requirement cannot generally be satisfied without special measures. First sources of error are attributable to guidance of the arm, which must be machined very accurately, since rotation about the longitudinal axis of the guide column have an effect, enhanced by the length of the arm, on the work-centering position of the tailstock. Secondly, thermal effects can cause changes in length of the arm, with resultant offsetting displacement of the center point. Another source of error resides in the mounting of the center rest to the machine.
Center rests form a fixed part of machines on which they are developed. If they are removably attached, then, after attachment or reattachment, the tailstock must be set accurately with its work-engaging center in alignment with the rotary axis of the machining or measuring device, and guidance of the tailstock arm must be aligned parallel to this axis. Such adjustment work requires a relatively great amount of time and shortens the useful life of the machine.
Universal machining and measuring machines are known which are not used exclusively for the working of cylindrical workpieces. In this case, a center rest (which is required only for workpieces of cylindrical symmetry) greatly limits the work area of the machine when it is not required, since the tailstock arm interferes with the path of travel of movable carriages of the machine. On the other hand, removal and reattachment of the center rest makes the above-mentioned adjustment work necessary.